Storage system

In a storage system having a plurality of virtualization apparatuses (11) that allocate a storage area which a storage device (13) has, form a plurality of virtual volumes (100), and process input-output from a host processor (12) to one of the virtual volumes (100), a request for completing all the input-output being processed that is received from the host processor (12) and temporarily holding the input-output processing received subsequently is issued to the plurality of virtualization apparatuses (11), when a completion report of the processing of the input-output in response to the request isreceivedfromthepluralityofvirtualizationapparatuses (11) and the completion report is received from all the virtualization apparatuses (11) to which the request was issued, an instruction of an allocation change of the storage area of the storage device (13) is sent to all the virtualization apparatuses (11), andwhen the completion report of the allocation change is received from all the virtualization apparatuses (11), the storage area of the storage device (13) is allocated to the virtual volume by sending an instruction for releasing a state of the input-output held temporarily to all the virtualization apparatuses (11).